In order to reduce the size, as well as increase efficiency of a power distribution assembly, conductive buss plates are employed having lateral direct interconnections to high current switching devices, thereby mitigating the traditional use of hard wiring and associated bulky cable harnesses. The buss plate is designed to be mounted to, and within, an enclosure whereby the components are then attached to the plate in such a manner as to complete the power supply circuit. Accordingly, buss plates lower the manufacturing costs by decreasing assembly time, as well as material costs. Furthermore, the flexibility of buss plates provide for a variety of form factors to accommodate obstacles inherent within the assembly, such as transformers, heat sinks, circuit breakers and the like.
Additionally, there are significant technical and functional advantages to the use of buss plates over hard wires. For example, a determining factor in mitigating electrical noise is to reduce circuit inductance while increasing the capacitance. Accordingly, the use of relatively thin parallel conductive buss plates, having a dielectric laminate as a substrate, has a tendency to minimize the effect of inductance by increasing the capacitance between electrical circuit planes. Laminated buss plates are important as well for the reduction of power circuit inductance to reduce transient voltages and to control parasitic oscillations when using high current insulated gate bi-polar transistor (IGBT) modules.
More specifically, it has been found that a PDA consisting of conductive buss plates, made from fabricated copper adhered onto a thin dielectric material and then sandwiched together to form a power buss circuit board, provides for both the mechanical mounting and electrical connectivity of components such as filter capacitors and semiconductor switching devices, for example IGBT's. The mechanical/electrical connection points or vias, are interposed between the conductive surfaces within the insulated buss plates. The components are secured with a fastener and a bushing or embossed conductive surface, the fastener passing through and contacting one or more of the plates and subsequently threading directly into a component. Notably, each connection through hole within the buss plate requires a copper bushing that is generally soldered into place, or alternatively an embossed surface, so as to be in direct alignment with the through hole. The copper bushing may be in the form of a flat-sided, washer-like component, and in one embodiment may also include a star-shaped locking washer to prevent problems with the backing-out or reversing of the fastener.
As mentioned, it is generally necessary to incorporate and retain a large bushing or washer around each connection through hole in the buss plate prior to assembly. However, the present practice requires the bushing to be pre-assembled to the buss plate by soldering, or welding the bushing to the buss plate about a hole in order to retain the bushing in position during the assembly operation. This requires a solder reflow, or similar process, to ensure that a plurality of connection points, have bushings therein, are simultaneously aligned for subsequent assembly. This soldering process is complex and in some cases has proven to be counterproductive and detrimental to providing a solid and reliable connection due to; (i) assembly alignment issues, (ii) thermal distortion introduced in the assembled components, (iii) compromised co-planarity, and (iv) increased softness of the bushing material. The soldering process also often results in corrosion due to the use of a fluxing agent, which interferes with a “hard” bushing to buss plate connection.
In order to solve the above-described problems, the present invention is directed to the assembly of a buss plate board connecting system comprising an electrically conductive buss plate having a plurality of through holes, a bushing disposed inline to a through hole having a hole therein coinciding with a hole in the buss plate and a compressible cylindrical retainer inserted within the hole of the bushing and the buss plate through hole so as to retain the bushing in general alignment with the buss plate through hole.
One object of the present invention is to provide an electro-mechanical inner connective means between buss plates and components that will overcome the above-described problems associated with pre-soldering the bushing in place by perpetually eliminating the bushing to buss plate soldering process with the use of a cylindrical spring bushing or retainer.
In accordance with a further aspect of the present invention, there is provided a compressible cylindrical retainer that mechanically retains the bushing in order to facilitate ease of assembly, whereby the bushing is permitted nominal movement to compensate for hole tolerances and offsets.
The assembly of the buss board structure, according to the present invention, can be greatly simplified by eliminating the necessity to align and solder the bushing onto the board prior to assembly. In effect, the bushing in the completed assembly is fundamentally secured to the buss board by the fastener and not the solder. Under the present process soldering of the bushing serves primarily as an interim means for the positioning of the bushing to assist in the assembly process, although in low-voltage applications the solder may assist with electrical conductivity. Once assembled there is little residual benefit and, in fact, soldering is all too often counterproductive and detrimental to a reliable connection due to the potential for; (i) misalignment, (ii) thermal distortion, (iii) corrosion and (iv) a weakened connection due to the metal fatigue from heating. Breaking or fracture of the soldered joints is even observed during the buss plane assembly process.
Additionally, by virtue of accumulated tolerances within the soldering process, the mechanical alignment of the bushing within the aperture of the first buss plate may not conform directly to the vertical axis of the mating buss plate or component because a bushing solder in place, unlike a cylindrical retainer, is incapable of yielding to compensate for coplanar alignment errors. As previously discussed the bushing must be in total contact in order to effectively conduct the high currents because contact surface area or diameter of the bushing is a function of the peak current capacity of the connection, expressed in circular mills. Consequently, one amp requires an area of approximately 400 circular mills of the bushing where a circular mill is defined as the square of the diameter of an equivalent round conductor expressed in units of 10−3 inches. Accordingly a skewed bushing will significantly decrease the contact area and therefore increase the connection resistance resulting in loss of power due to the generated heat. Therefore an objective in designing a high current power supply buss is to maintain low contact resistance by maximizing the contact surface area so as to provide only a nominal voltage drop and minimize the associated resistive heating effect in watts (W) at the connection point according to Ohm's Law where R=V2/W and W=I2×R.
The embodiment described and disclosed herein details aspects of the present invention in accordance with an interlocking cylindrical retainer/bushing assembly providing for an interim means to reliably locate the bushing onto the buss plate until a positive connection is established with a fastener passing through the buss plate and into an electrical component. The present invention therefore provides for significantly improved ease of assembly, improved reliability and reduced cost of manufacture. It is further contemplated that aspects of the disclosed embodiments permit the use of various and alternative materials, where the bushing is constructed of a material different from the buss plate and which may be plated or formed from a highly-conductive material.
In accordance with an embodiment disclosed herein, there is provided a buss plate connecting system comprising: an electrically conductive buss plate having a plurality of through holes; a bushing, disposed inline to a through hole within the buss plate having a hole therein said bushing coinciding with the through hole in the buss plate; and a circumferentially compressible cylindrical retainer inserted within the hole of the bushing and the through hole of the buss plate, to retain said bushing in general alignment with said buss plate through hole.
In accordance with another embodiment disclosed herein there is provided a buss plate connecting system having a first and second bushing, comprising: an electrically conductive buss plate having a plurality of through holes; a first and second bushing disposed inline to a through hole within the buss plate and having a hole therein said bushings coinciding with the hole in the buss plate; and a circumferentially compressible cylindrical retainer inserted within the hole of the first bushing, the buss plate through hole and the hole of the second bushing, to retain said bushings in general alignment with said buss plate through hole.
In accordance with a further embodiment disclosed herein there is provided a method of interconnecting a buss plate with a component comprising: placing at least one bushing in proximity to a through hole within a buss plate; compressing a cylindrical retainer, having a longitudinal slot, to reduce its diameter; inserting said retainer through a hole in the bushing and the buss plate through hole; and releasing the retainer thereby urging the bushing in alignment with said through hole in said buss plate.